Bring water to the boil in a saucepan and stir in mocha powder. Bring the mocha to the boil briefly and set aside so that the powder can settle.
Transfer the mocha to another pot. Coarsely chop the chocolate and melt it in the mocha.
Take a few spoonfuls of the cream and mix with the cornflour in a small bowl. Add the rest of the cream with the sugar and salt to the chocolate mocha and bring to the boil. Stir in the cream and starch mixture and allow the cream to thicken while stirring.
Remove the pan from the heat and allow the cream to cool briefly. Whisk the egg yolks and stir into the chocolate mocha cream. Cover with cling film so that the cream does not get a skin.
Grease the muffin tin and preheat the oven to 180 °C top/bottom heat.
Melt the butter in a small saucepan. Cut a slice of filo pastry into quarters so that you have approx. 10 x 15 cm rectangles. Brush each slice with a little butter and lay them on top of each other, staggered.
Carefully place the cupcakes into the cavities in the greased tin. Spread approx. 2-3 tbsp. of the chocolate mocha cream on each and bake in the oven for approx. 20 minutes until crisp.
Leave the chocolate mocha filo tarts to cool, remove from the tin and garnish as desired with berries and dust with cocoa.